Scientists knew that Earth is crying since the 1970s, but they've just recently been able to listen to it.

The "cry" is actually a series of chirp and whistles created by solar wind particles colliding with the Earth's magnetic field high above the planet, and is called the Auroral Kilometric Radiation (AKR):

The Auroral Kilometric Radiation (AKR), as it is called, is beamed out in a narrow plane, as if someone had put a mask over a torch and left a slit for the radiation to escape.

This flat beam could be detected by aliens who've figured this process out, the researchers say. The knowledge could also be used by Earth's astronomers to detect planets around other stars, if they can build a new radio telescope big enough for the search. They could also learn more about Jupiter and Saturn by studying AKR, which should emit from the auroral activity on those worlds, too.